Because lithium secondary batteries have higher energy densities compared with other secondary batteries such as nickel-cadmium batteries and nickel-hydrogen batteries and they can be operated at higher potentials, they are widely used as power sources for small information devices such as mobile telephones and notebook personal computers. Further, in recent years, lithium secondary batteries have been able to be more easily reduced in size and weight, resulting in growing demands for larger scale applications such as hybrid vehicles and electric vehicles, or fixed domestic storage batteries.
All of these lithium secondary batteries include, as the main structural elements, a cathode and an anode containing a material that can reversibly store and release lithium, an electrolyte containing a lithium ion conductor dissolved in a non-aqueous organic solvent, and a separator. Of these structural elements, examples of oxides that are used as the cathode material include lithium cobalt oxide (LiCoO2), lithium manganese oxide (LiMn2O4), lithium nickel oxide (LiNiO2), and lithium nickel-cobalt-manganese oxide (LiNi1/3Co1/3Mn1/3O2).
On the other hand, in order to enable more widespread use in large-scale applications, the use of the scarce element cobalt for the cathode material is undesirable from resource and cost perspectives, and a high-capacity cathode material that does not use cobalt as a constituent element would be preferable.
Lithium manganese oxide cathode materials have a voltage of about 3 to 4 V relative to lithium as a result of a lithium extraction/insertion reaction, and materials having various crystal structures have been investigated as cathode materials. Among these materials, spinel lithium manganese oxide LiMn2O4 has a potential plateau in the region of 4 V relative to lithium, and exhibits favorable reversibility of the lithium extraction/insertion reaction, and is consequently currently being used as a practical material. However, the capacity per weight of oxide is only about 100 mA/g, making it unsuitable for high-capacity lithium secondary batteries.
On the other hand, lithium manganese oxide, which has a layered rock-salt structure similar to that of lithium cobalt oxide and the like, has also been investigated as a high-capacity cathode material.
However, it is well known that in the case of lithium manganese oxide, the charge/discharge curves change as the number of charge/discharge cycles increase, and gradually change to charge/discharge curves characteristic of a spinel phase.
In contrast, lithium nickel-titanium-manganese oxide, which has a layered rock-salt structure capable of a high capacity of about 250 mAh/g, has been investigated as a composition having a structure that is resistant to spinelization upon repeated charge/discharge, and it is clear that particularly when the ratio of Ni:Ti:Mn is close to 1:1:8, changes in the charge/discharge curves are small. However, some changes in the charge/discharge curves upon cycling still remain (Patent Document 1, Non-Patent Document 1).
Further, it has been reported that in Ni and Mn-based cathode materials having layered rock-salt structures, the cycle characteristics can be improved by substituting with Mg, Na or Al or the like.
Although these element substitutions have been shown to have some effect in improving the cycle characteristics, reduction in the capacity remains a problem.
On the other hand, among systems having a layered rock-salt structure similar to that of lithium cobalt oxide and the like, lithium nickel-cobalt-manganese oxides or lithium nickel-manganese oxides formed from lithium-excess compositions have also been investigated as high-capacity cathode materials.
Whereas normal layered rock-salt structures have a hexagonal system (trigonal system) space group R-3m crystal structure, layered rock-salt structures having a lithium-excess composition belong to the space group C2/m of a lower symmetry monoclinic system, and in a powder X-ray diffraction pattern using CuKα radiation, yield a diffraction pattern in which the 2θ angle is in a region from 20 to 35 degrees, consistent with this reduction in symmetry. Moreover, in crystal structure analyses using the Rietveld method or the like, analysis can be performed using a crystal structure model in which the lithium occupancy occurs in a transition metal layer.
In particular, lithium nickel-manganese oxide having a lithium-excess composition is expected to yield a high capacity of up to 300 mAh/g, and is therefore being actively investigated (Patent Document 2).
However, it is known that the charge/discharge curves change as the number of charge/discharge cycles increases, gradually approaching the charge/discharge curves characteristic of a spinel phase, and the changes in operating voltage are a practical problem.
In order to address this problem, the synthesis of LixNi1/4Mn3/4−yTiyO2, in which a portion of the manganese has been substituted with titanium has been reported to have an effect in enhancing the stability of the crystal structure, but although some effect is achieved in terms of the changes in the charge/discharge curves, it has not lead to a fundamental resolution of the problem (Patent Document 3).
These changes in the charge/discharge curves that accompany spinelization cause a reduction in the charge of the lithium layers in the charged state, resulting in increased structural instability, and are therefore thought to be due to the migration of transition metal ions from the transition metal layers.
Accordingly, with only titanium substitution, completely suppressing the changes in the shapes of the charge/discharge curves that accompany cycling is difficult, and therefore by further optimizing the composition of the titanium-substituted material, and introducing a cation having strong chemical bonding into the lithium layers of the layered rock-salt structure, further improvements in the structural stability of the lithium layers are expected.
In accordance with this principle, the substitution of magnesium into lithium manganese-titanium oxides or lithium manganese-iron oxides having lithium-excess compositions has been investigated (Patent Document 4).
As outlined above, among systems having a lithium-excess layered rock-salt structure, which are expected to provide high capacities as cathode materials, for lithium manganese-titanium oxides and lithium manganese-iron oxides, magnesium substitution into the lithium layers of the layered rock-salt structure has been investigated as a way of suppressing changes in the charge/discharge curves that accompany charge/discharge cycling. However, magnesium ions have an ionic radius that is similar to those of transition metal ions and lithium ions, and because substitution occurs in both the lithium layers and the transition metal layers, although some effect can be confirmed in terms of improved reversibility, a problem arises in that this results in a reduction in the capacity. Due to these circumstances, this type of magnesium substitution of lithium-nickel-manganese composite oxides, lithium-nickel-cobalt-manganese composite oxides and lithium-nickel-titanium-manganese composite oxides having lithium-excess layered rock-salt structures has not been investigated.
In general, it is known that when the lithium occupancy of the lithium layers decreases as charging proceeds, the interlayer distance of the lithium layers tends to widen. Accordingly, in the charged state, the substitution of elements having a larger ionic radius, which exhibit an effect with larger interlayer distances, should be effective in suppressing crystal structure change. As a result, substitution using magnesium and calcium, or substitution using only calcium, should be much more effective than substitution using only magnesium, but up until now, the substitution of calcium ions into the lithium layers of a lithium-excess composition has been assumed to be problematic due to the large difference in ionic radii, and therefore there are no reports of such substitution in the existing literature.
Moreover, it is also well known that in material systems having a lithium-excess composition, during the initial charging reaction, in addition to the extraction reaction of lithium from interlayer spaces, oxygen extraction and migration of transition metals through the crystal structure also occur (Non-Patent Document 4).
This oxygen extraction reaction is known to generate a potential plateau at about 4.5 V relative to lithium during initial charging, and because this reaction is necessary for achieving a high capacity, the fact that the irreversible capacity is large, as indicated by a small initial discharge capacity relative to the initial charging capacity, is a practical problem (for example, see the charge curve for the 1st cycle in FIG. 4(c) of Non-Patent Document 2).
Further, even in those cases where a high initial discharge capacity of about 250 mAh/g is obtained, it is known that as cycling is repeated, a change in the crystal structure of the material causes a large decrease in the discharge voltage, and a marked reduction in the capacity also occurs.
Accordingly, when material systems having a lithium-excess composition are used in actual battery systems, electrochemical activation of the electrodes, including this type of crystal structure change and chemical composition change, must be performed, and for example, a stepwise charging method in which the upper limit voltage is increased with each cycle has been proposed (Non-Patent Document 4).
However, even with this stepwise charging method, the upper limit voltage must be set to a high voltage of 4.8 V to achieve high capacity, meaning another problem arises in that, with current battery systems, a measure for suppressing oxidative decomposition of the electrolyte is also required.
Accordingly, rather than requiring this type of electrode electrochemical activation method, synthesis of a material for which an oxygen extraction reaction and crystal structure change do not occur following material synthesis, or can be suppressed as far as possible, would not require the steps associated with the electrochemical activation treatment, which would be very desirable.